prune unused values based on behavior

remove the hard-coded types from PruneUnusedValuesTransformer

terraform/node_local.go

@@ -22,8 +22,14 @@ var (
 	_ GraphNodeReferenceable     = (*NodePlannableLocal)(nil)
 	_ GraphNodeReferencer        = (*NodePlannableLocal)(nil)
 	_ GraphNodeDynamicExpandable = (*NodePlannableLocal)(nil)
+	_ graphNodeTemporaryValue    = (*NodePlannableLocal)(nil)
 )
 
+// graphNodeTemporaryValue
+func (n *NodePlannableLocal) temporaryValue() bool {
+	return true
+}
+
 func (n *NodePlannableLocal) Name() string {
 	path := n.Module.String()
 	addr := n.Addr.String()
@@ -83,9 +89,15 @@ var (
 	_ GraphNodeReferenceable  = (*NodeLocal)(nil)
 	_ GraphNodeReferencer     = (*NodeLocal)(nil)
 	_ GraphNodeEvalable       = (*NodeLocal)(nil)
+	_ graphNodeTemporaryValue = (*NodeLocal)(nil)
 	_ dag.GraphNodeDotter     = (*NodeLocal)(nil)
 )
 
+// graphNodeTemporaryValue
+func (n *NodeLocal) temporaryValue() bool {
+	return true
+}
+
 func (n *NodeLocal) Name() string {
 	return n.Addr.String()
 }

terraform/node_module_variable.go

@@ -25,9 +25,14 @@ var (
 	_ GraphNodeReferenceOutside  = (*NodePlannableModuleVariable)(nil)
 	_ GraphNodeReferenceable     = (*NodePlannableModuleVariable)(nil)
 	_ GraphNodeReferencer        = (*NodePlannableModuleVariable)(nil)
+	_ graphNodeTemporaryValue    = (*NodePlannableModuleVariable)(nil)
 	_ RemovableIfNotTargeted     = (*NodePlannableModuleVariable)(nil)
 )
 
+func (n *NodePlannableModuleVariable) temporaryValue() bool {
+	return true
+}
+
 func (n *NodePlannableModuleVariable) DynamicExpand(ctx EvalContext) (*Graph, error) {
 	var g Graph
 	expander := ctx.InstanceExpander()
@@ -120,9 +125,14 @@ var (
 	_ GraphNodeReferenceable    = (*NodeApplyableModuleVariable)(nil)
 	_ GraphNodeReferencer       = (*NodeApplyableModuleVariable)(nil)
 	_ GraphNodeEvalable         = (*NodeApplyableModuleVariable)(nil)
+	_ graphNodeTemporaryValue   = (*NodeApplyableModuleVariable)(nil)
 	_ dag.GraphNodeDotter       = (*NodeApplyableModuleVariable)(nil)
 )
 
+func (n *NodeApplyableModuleVariable) temporaryValue() bool {
+	return true
+}
+
 func (n *NodeApplyableModuleVariable) Name() string {
 	return n.Addr.String()
 }

terraform/node_output.go

@@ -23,8 +23,14 @@ var (
 	_ GraphNodeReferenceable     = (*NodePlannableOutput)(nil)
 	_ GraphNodeReferencer        = (*NodePlannableOutput)(nil)
 	_ GraphNodeDynamicExpandable = (*NodePlannableOutput)(nil)
+	_ graphNodeTemporaryValue    = (*NodeApplyableOutput)(nil)
 )
 
+func (n *NodePlannableOutput) temporaryValue() bool {
+	// this must always be evaluated if it is a root module output
+	return !n.Module.IsRoot()
+}
+
 func (n *NodePlannableOutput) DynamicExpand(ctx EvalContext) (*Graph, error) {
 	var g Graph
 	expander := ctx.InstanceExpander()
@@ -115,9 +121,15 @@ var (
 	_ GraphNodeReferencer       = (*NodeApplyableOutput)(nil)
 	_ GraphNodeReferenceOutside = (*NodeApplyableOutput)(nil)
 	_ GraphNodeEvalable         = (*NodeApplyableOutput)(nil)
+	_ graphNodeTemporaryValue   = (*NodeApplyableOutput)(nil)
 	_ dag.GraphNodeDotter       = (*NodeApplyableOutput)(nil)
 )
 
+func (n *NodeApplyableOutput) temporaryValue() bool {
+	// this must always be evaluated if it is a root module output
+	return !n.Addr.Module.IsRoot()
+}
+
 func (n *NodeApplyableOutput) Name() string {
 	return n.Addr.String()
 }

terraform/node_value.go

@@ -0,0 +1,10 @@
+package terraform
+
+// graphNodeTemporaryValue is implemented by nodes that may represent temporary
+// values, which are those not saved to the state file. This includes locals,
+// variables, and non-root outputs.
+// A boolean return value allows a node which may need to be saved to
+// conditionally do so.
+type graphNodeTemporaryValue interface {
+	temporaryValue() bool
+}

terraform/transform_reference.go

@@ -185,63 +185,40 @@ type PruneUnusedValuesTransformer struct {
 	Destroy bool
 }
 
-// FIXME: outputs will now  have module expansion root nodes dependent on them,
-// so we need to filter those out too. Might be a good time to decouple this
-// from concrete types, and implement some interfaces.
 func (t *PruneUnusedValuesTransformer) Transform(g *Graph) error {
 	// Pruning a value can effect previously checked edges, so loop until there
 	// are no more changes.
 	for removed := 0; ; removed = 0 {
 		for _, v := range g.Vertices() {
+			// we're only concerned with values that don't need to be saved in state
 			switch v := v.(type) {
-			case *NodeApplyableOutput:
+			case graphNodeTemporaryValue:
-				// If we're not certain this is a full destroy, we need to keep any
+				if !v.temporaryValue() {
-				// root module outputs
-				if v.Addr.Module.IsRoot() && !t.Destroy {
 					continue
 				}
-			case *NodePlannableOutput:
-				// Have similar guardrails for plannable outputs as applyable above
-				if v.Module.IsRoot() && !t.Destroy {
-					continue
-				}
-			case *NodeLocal, *NodeApplyableModuleVariable, *NodePlannableModuleVariable:
-				// OK
 			default:
-				// We're only concerned with variables, locals and outputs
 				continue
 			}
 
 			dependants := g.UpEdges(v)
 
-			switch dependants.Len() {
+			// any referencers in the dependents means we need to keep this
-			case 0:
+			// value for evaluation
-				// nothing at all depends on this
+			removable := true
+			for _, d := range dependants.List() {
+				if _, ok := d.(GraphNodeReferencer); ok {
+					removable = false
+					break
+				}
+			}
+
+			if removable {
 				log.Printf("[TRACE] PruneUnusedValuesTransformer: removing unused value %s", dag.VertexName(v))
 				g.Remove(v)
 				removed++
-			default:
-				// because an output's destroy node always depends on the output,
-				// we need to check for the case of a single destroy node.
-				removable := true
-			SEARCH:
-				for _, d := range dependants.List() {
-					switch d.(type) {
-					case *NodeDestroyableOutput, *nodeCloseModule:
-						//pass
-					default:
-						removable = false
-						break SEARCH
-					}
-				}
-
-				if removable {
-					log.Printf("[TRACE] PruneUnusedValuesTransformer: removing unused value %s", dag.VertexName(v))
-					g.Remove(v)
-					removed++
-				}
 			}
 		}
+
 		if removed == 0 {
 			break
 		}